As shown in FIG. 1, a wireless communication system 10 comprises elements such as client terminal or mobile station 12 and base stations 14. Other network devices which may be employed, such as a mobile switching center, are not shown. In some wireless communication systems there may be only one base station and many client terminals while in some other communication systems such as cellular wireless communication systems there are multiple base stations and a large number of client terminals communicating with each base station.
As illustrated, the communication path from the base station (BS) to the client terminal direction is referred to herein as the downlink (DL) and the communication path from the client terminal to the base station direction is referred to herein as the uplink (UL). In some wireless communication systems the client terminal or mobile station (MS) communicates with the BS in both DL and UL directions. For instance, this is the case in cellular telephone systems. In other wireless communication systems the client terminal communicates with the base stations in only one direction, usually the DL. This may occur in applications such as paging.
The base station to which the client terminal is communicating with is referred as the serving base station. In some wireless communication systems the serving base station is normally referred as the serving cell. While in practice a cell may include one or more base stations, a distinction is not made between a base station and a cell, and such terms may be used interchangeably herein. The base stations that are in the vicinity of the serving base station are called neighbor cell base stations. Similarly, in some wireless communication systems a neighbor base station is normally referred as a neighbor cell.
Duplexing refers to the ability to provide bidirectional communication in a system, i.e., from base station to client terminals (DL) and from client terminals to base station (UL). There are different methods for providing bidirectional communication. One of the commonly used duplexing method is the Frequency Division Duplexing (FDD). In FDD wireless communication systems, two different frequencies, one for DL and another for UL are used for communication. In FDD wireless communication system, the client terminals may be receiving and transmitting simultaneously.
Another commonly used method is the Time Division Duplexing (TDD). In TDD based wireless communication system, the same exact frequency is used for communication in both DL and UL. In TDD wireless communication systems, the client terminals may be either receiving or transmitting but not both simultaneously. The use of the RF channel for DL and UL may alternate on periodic basis. For example, in every 5 ms time duration, during the first half, the RF channel may be used for DL and during the second half, the RF channel may be used for UL. In some communication systems the time duration for which the RF channel is used for DL and UL may be adjustable and may be changed dynamically.
Yet another commonly used duplexing method is half-duplex FDD (H-FDD). In this method, different frequencies are used for DL and UL but the client terminals may not perform receive and transmit at the same time. Similar to TDD wireless communication system, the client terminal using H-FDD method must periodically switch between DL and UL operation. All three duplexing methods are illustrated in FIG. 2.
In many TDD wireless communication systems, normally the communication between the base station and client terminals is organized into frames as shown in FIG. 3. The frame duration may be different for different communication systems and normally of the order of milliseconds. For a given communication system the frame duration may be fixed. For example, the frame duration may be 5 milliseconds.
In a TDD wireless communication system, the frame is divided into a DL subframe and a UL subframe. In TDD wireless communication systems, the communication from base station to the client terminal (DL) direction takes place during the DL subframe and the communication from client terminal to network (UL) direction takes place during UL subframe on the same RF channel. When a client terminal is powered on, it may not have information about the frame timing of the base station. The client terminal may first perform power scan and select a suitable RF channel for performing synchronization with the base station timing. Therefore, in general the power scan may be performed by the client terminal when its timing is not aligned with that of the base station.
As shown in FIG. 4, client terminal/MS 12 typically contains a baseband subsystem 16 and a radio frequency (RF) subsystem 18. Memory 20, such as an external memory, is shown connected to the baseband subsystem 16. The baseband subsystem 16 normally includes a micro controller unit (MCU) 22, a signal processing unit (SPU) 24, data converters 26, peripherals 28, power management 30, and memory 32 as shown in FIG. 5. The SPU 24 may be a digital signal processor (DSP), hardware (HW) accelerators, co-processors or a combination of the above. Normally the overall control of the baseband subsystem 16 is performed by software running on the MCU 22 and the processing of signals is done by the SPU 24.